The present invention relates to lubricating oil compositions which demonstrate improved fuel economy. These fuel economizing lubricants employ an overbased sulfonate detergent having a total base number greater than about 450 and at least one other additive which includes a succinimide type dispersant.
While motor vehicle manufacturers continue to seek improved fuel economy through engine design; new approaches in formulating engine oils have played an important role in improving fuel economy and have resulted in improved emission characteristics of motor vehicles. Lubricant optimization is especially preferred over engine hardware changes, due to its comparative lower cost per unit in fuel efficiency and possibility for backward compatibility with older engines. Therefore, formulators are under continued pressure to develop engine oils and additive packages which take advantage of new performance basestocks and additive blends which demonstrate better fuel efficiency, oxidative stability, volatility, and improved viscosity index (to name a few characteristics) over conventional formulations. To improve fuel efficiency, there has been a drive to use lower viscosity engine oils, which often requires new additive package formulations. High on the list of requirements for these new formulated engine oil specifications are those employing components which improve the frictional properties of the lubricating oil composition. In this case, the additive system design is the crucial factor and close attention must be focused on the additive/additive and additive/base fluid interactions.
Engine oil acts as a lubricant between moving engine parts at various conditions of load, speed and temperature. Hence, the various engine components experience different combinations of boundary layer, mixed and (elasto) hydrodynamic regimes of lubrication; with the largest frictional losses at piston liner/piston ring interface and a smaller part by the bearing and valve train. To reduce the energy losses due to friction of the various parts and to prevent engine wear, additives are incorporated into the engine oil such as friction modifiers, anti-wear agents, and antioxidants; the latter of which tend to lengthen the effect of the afore mentioned additives. Also to reduce the hydrodynamic friction in the piston/cylinder, the viscosity of engine oils has been lowered which has increased the dependence of friction modifiers to offset the new boundary layer regime. Hence, a vast amount of effort has focused on the interaction of oil viscosity with various friction modifiers to improve fuel economy.
Friction modifiers have been around for several years for application in limited slip gear oils, automatic transmission fluids, slideway lubricants and multipurpose tractor fluids. With the desire for increased fuel economy, friction modifiers have been added to automotive crankcase lubricants and several are known in the art. They generally operate at boundary layer conditions at temperatures where anti-wear and extreme pressure additives are not yet reactive by forming a thin mono-molecular layers of physically adsorbed polar oil-soluble products or reaction layers which exhibit a significantly lower friction compared to typical anti-wear or extreme pressure agents. However, under more severe conditions and in mixed lubrication regime these friction modifiers are added with an anti-wear or extreme pressure agent. The most common type is a zinc dithiophosphate (ZnDTP or ZDDP), which, due to emissions considerations, has been reduced in concentration in many current formulations.
Anti-wear, extreme pressure, anti-corrosion, and friction modifiers; as well as detergents and dispersants, are all polar additives which have an affinity to metal surfaces and can compete for the active metal surface site, or interact with each other. For example, anti-wear agents such as ZnDTP and ZnDTC protect closely approaching metal surfaces from asperities from damaging the opposite surface. These films are semi-plastic and are difficult to shear off, so that under shearing conditions, their coefficient of friction is generally high. Conversely, a friction modifier generally operates by building an orderly and closely packed arrays of multi-molecule layers which are attracted to the metal surface via their polar heads and aligned to each other via Van der Waal forces. Therefore, when surface active agents such as anti-wear agents ZnDTP, a friction modifier, dispersant or detergent are added to a lubricating oil, the adsorption of the anti-wear agent is reduced by the competitive adsorption of the other agents. The resulting protective film formation can be retarded or eliminated; thus, decreasing the overall available power and engine efficiency.
Dispersants and detergents are widely known in the art, and are generally employed to keep sludge, carbon and other deposit precursors derived from partial oxidation of the fuel or lubricating oils suspended in the oil. In addition, detergents function to neutralize potentially corrosive acids and to contribute to cleanliness. For engine oils this is primarily to neutralize acidic by products of combustion, oxidation or decomposition and thereby reduce the amount of corrosive wear and also to keep the pistons and other high temperature surfaces clean of deposits. However, as stated above, dispersants and detergents are generally polar molecules which can adversely interact with other functional additives. For example, overbased sulfonates are also known to act as a pro-oxidant and degrade antioxidant performance. Outside of their contribution to viscosity of the final lubricating composition, the degree of overbasing of a detergent was not thought to have any effect related to fuel economy.
Accordingly, the selection of components and interactions between them is of major concern and beneficial interactions or new properties and improvements resulting therefrom, are not expected or possible to anticipate. Thus when discovered, especially when additives are used which exhibit a dual benefit not appreciated in the art, clearly advances the art. While significant improvements in fuel economy have been achieved since 1987 (with EC-I, GF-1, GF-2 and GF-3 compliant lubricants) proposed GF-4 oils and future standards will prove further developments are necessary and paradigm shifts to additive formulations are needed. In the present invention, a surprisingly significant effect on fuel economy has been attributed to detergent/dispersant selection.
The present invention provides a lubricating composition comprising a major amount of an oil of lubricating viscosity, an oil soluble overbased alkaline earth alkyl aryl sulfonate detergent having a total base number (TBN) of about 450 to 550, and an alkenyl succinimide dispersant derived from a 450 to 3000 average molecular weight polyalkylene. Formulated lubricating oil employing such high TBN overbased sulfonates have shown improved properties, especially in fuel economy when compared to conventional overbased metal detergents. Accordingly the use of such lubricants can be applied in engine oil formulations, crankcase formulations, and gear oil formulations for improving fuel economy.
Accordingly, one embodiment is directed to a formulated lubricating composition. One such formulation for internal combustion engines comprises: (a) a major amount of a base oil of lubricating viscosity; (b) 0.5% to 10% of an overbased alkaline earth metal, preferably calcium, alkyl aryl sulfonate detergent having a total base number (TBN) of about 450 to 550; (c) 1% to 20% of an alkenyl succinimide dispersant, preferably a carbonate treated alkenyl succinimide, derived from a 450 to 3000 average molecular weight polyalkylene; (d) 0.05% to 1.0% of a friction modifier; and (e) 0.1% to 2.0% of a zinc dialkyldithiophosphate; wherein the percent additive is based upon weight percent of the lubricating oil composition.
In another aspect, this invention is directed to a concentrate comprising from about 10% to 80% of an overbased alkaline earth metal, preferably calcium, alkyl aryl sulfonate detergent having a total base number (TBN) of about 450 to 550 and from about 20% to 60% of an alkenyl succinimide dispersant, preferably a carbonate treated alkenyl succinimide, derived from a 450 to 3000 average molecular weight polyalkylene and about 1% to 10% of a compatible organic liquid diluent.
Lubricating oil compositions employing an overbased sulfonate detergent having a total base number greater than about 450 and at least one other additive which includes a succinimide type dispersant have shown improved fuel economy over conventional detergents. Accordingly, this invention is directed to a method for improving fuel economy of an internal combustion engine, preferably gasoline, comprising operating said engine with a lubricating composition comprising a major amount of an oil of lubricating viscosity and a fuel economizing amount of an oil soluble overbased alkaline earth alkyl aryl sulfonate detergent having a TBN greater than 450 and a alkenyl succinimide dispersant derived from a 450 to 3000 average molecular weight polyalkylene. One such method for determining fuel economy can be measured by the Sequence VIB test using a reference oil.
Among other factors, the present invention is based upon the surprising discovery that high overbased (greater than 450 TBN) alkyl aryl sulfonate detergents provide an improvement in fuel economy over lower overbased alkyl aryl sulfonate detergents, and in comparison with other conventional detergent chemistries. More specifically, lubricating compositions and formulated engine oils employing a polyalkylene succinimide dispersant and a sulfonate detergent having a TBN greater than about 450 have been shown to improve fuel economy. Therefore, employing such a lubricating composition in an engine oil application, gear oil application or other application requiring lubrication, can lead to an improvement in overall fuel economy.